1. Field of the Invention
This invention relates generally to structural fasteners, and more particularly relates to drive sockets on threaded male structural fasteners.
2. Description of the Related Art
Threaded male structural fasteners of the type to which this invention is directed include an axial drive socket, or wrench engaging recess, located either on a head (FIG. 1a) of the fastener or on the opposite end of the fastener (FIG. 1b). The drive socket (FIG. 1c) includes a plurality of substantially equally spaced and sized flat surfaces which receive a mating wrench type key, or wrench key. The drive socket may be hexagonally shaped to receive a hexagonal Allen wrench type key, or hex-key driver. During use, an end of the wrench key is inserted into the drive socket in the male fastener and is either held in place to permit a threaded female fastener, such as a nut, to be screwed onto the threads of the male fastener, or the hex-key driver is rotated in order to screw the male fastener into the female fastener or other female threaded hole.
Clearance is required between a hex-key driver and the drive socket which the hex-key driver is inserted into. For example, the clearance between a 2.4 mm ( 3/32-inch) hexagonal drive socket and a 2.4 mm ( 3/32-inch) hex-key driver is 0.04 mm (0.0015 inch) all around. With this amount of clearance, the hex-key driver can rotate approximately three degrees before contacting the mating surfaces of the drive socket (FIG. 1d), resulting in the intersections, or corners, between the surfaces of the hex-key driver contacting the flat surfaces of the drive socket. With the corners between the surfaces of the hex-key driver providing the main driving force from the hex-key driver, high stress concentrations are placed on the corners of the hex-key driver, but there is very little material providing support for the corners. As a result, when a load that is sufficient to set the desired torque between the male threaded fastener and the female threaded fastener or part is applied to a hex-key driver during use, the material at the corners may shear off of the hex-key driver, leaving the hex-key driver in a stripped condition.
Another problem that can occur is that as the hex-key driver is rotated within the drive socket, the hex-key driver may become wedged within the socket, thus causing the hex-key driver to bind within the socket. After setting the desired torque between the male threaded fastener and the female threaded fastener, rotation of the hex-key driver must be reversed to break the bind between the hex-key driver and the socket.
Methods which have been developed to prevent the stripping of hex-key drivers during use include fabricating the hex-key drivers from stronger materials. Yet, making the hex-key driver stronger may lead to stripping the drive socket during high stress applications. Another method of preventing hex-key drivers from stripping includes making the drive sockets in the fasteners deeper. However, making the drive socket deeper may compromise tensile strength of the fastener through the area of the drive socket, especially on fasteners having the drive socket on the end of the fastener opposite the head (FIG. 1b).
One attempt to alleviate the problem of the hex-key drivers stripping within the drive sockets included reconfiguring the drive socket to include six rounded lobes of substantially equal radius, as depicted in FIG. 2a. The lobes are located substantially equidistant from the center of the fastener. Also, adjacent lobes are located substantially equidistant circumferentially from each other. Each of the lobes projects inward toward the center of the fastener. The embodiment also includes curved recesses located between and adjacent each lobe, thereby combining for a total of six recesses. Each of the recesses is blended into its adjacent lobes to form a substantially smooth transition between the lobes and the recesses.
Referring to FIG. 2b, when the hex-key driver is rotated, such as to drive the male threaded fastener relative to a female threaded fastener, a portion of the flats on the hex-key driver contact the lobes while the corners of the hex-key driver remain within the recesses and out of contact with the wall of the drive socket. One problem with the configuration of FIGS. 2a and 2b is that it enlarges the drive socket considerably, thereby weakening the fastener head.
Hence, those skilled in the art have recognized a need for a male threaded fastener having a drive socket that may receive high stress from a hex-key driver and reduce the tendency of the hex-key driver to strip or to bind within the drive socket. The need to configure the drive socket so that it substantially maintains the tensile strength of male threaded fasteners and the strength of the fastener heads has also been recognized. The present invention fulfills these needs and others.